By BRIAN NEARING Staff writer

Published 12:01 am, Saturday, September 3, 2011

KNOX -- Small earthquakes -- now up to more than two dozen and counting -- are rumbling again in the Helderbergs near Thacher Park and the village of Altamont.

The resumption of mini-quakes this month, after a quiet spell that lasted nearly two years, has prompted researchers from Columbia University to return with seismic equipment in a bid to better understand what is happening.

On Tuesday, a solar-powered seismic sensor was placed in an open field at the Emma Treadwell Thacher Nature Center next to Thompson's Lake State Park, said Won-Young Kim, a research professor at Columbia's Lamont-Doherty Earth Observatory.

Kim said it is hoped the sensor will provide clues to a recent spate of 26 quakes that started Aug. 22, the day before a massive earthquake centered in Virginia rattled the eastern seaboard, and continued through Aug. 28.

Centered about three miles west of Altamont, the largest of the quakes was 2.9 on the Richter scale. That is about the intensity that people can begin to feel, but is too little to cause any damage.

The tremors, centered between 11 to 15 miles below the surface, are about twice as deep as an average quake, Kim said. "This is one of the issues that we do not understand, what is going on at this depth. It is very strange."

The Aug. 27 quake prompted Gov. Andrew Cuomo to order an inspection of the New York Power Authority's Blenheim-Gilboa Dam in nearby Schoharie County. That inspection found no faults in the dam, according to an authority spokesman.

There have been periodic clusters of little earthquakes in Knox and nearby Berne -- 91 in total -- dating back to the 1980s, said Chuck Ver Straeten, a geologist with the State Museum and Geological Survey.

The most recent outbreak occurred between February 2009 and March 2010, when 37 quakes were recorded. In 1982, a dozen quakes were reported in Berne in a single three-hour period. Lamont Doherty researchers had installed four seismic monitors in the Berne area last summer and fall to further study the last spate of tremors, but the area became and remained quiet, so the sensors were later removed.

Ver Straeten said there likely is no reason to be concerned the quakes will increase in intensity.

He said the rocks where the quakes appear to be centered are likely a billion years old, or older, and could be linked to faults that run through the equally ancient bedrock of the Adirondacks south into the Helderbergs.

Two geologic fault lines, known as the McGregor-Saratoga and Hoffmans faults, run like underground scars northeast from the Albany County Hilltowns and Schoharie County, up through Saratoga County before snaking into the Adirondacks.